Queens of Crime never abdicate, which, in the case of Agatha Christie, meant that reviewers of her final books had to decide whether to be hypocritical or cruel. She was in her 80s and so out of touch with contemporary life that her descriptions of it had become embarrassing.

Luckily her successor PD James presents no such problems. At 85 she is still a working peer, more actively involved with the world than most authors. Her people, places and plots are as artful as ever and her dialogue, having always been uncolloquially correct and expletive-free, does not date. The Lighthouse is a whodunit which takes murder seriously; this writer never lets her readers forget that death is no joke. The earnest viewpoint is combined with playful use of the conventions of the mystery game, a formal pattern in which suspects come forward in turn, as the detectives scrupulously consider a series of clues and motives.

Unique settings are a PD James speciality, from a tower on a Dorset hilltop, a nuclear power station on the East Anglian coast and a particular London church to, in this book, an offshore island used as a high-security hideout for VIPs. Being hard or impossible to reach, an island setting for a murder mystery is the equivalent of the traditional snowed-in manor house, and as the late Lord Hardinge of Penshurst, who for decades was the most influential crime-fiction editor in Britain, once remarked, "most crime writers feel the need to write an 'island book' sooner or later."

Combe Island lies 12 miles off the north coast of Cornwall. It was once a pirates' lair, then a private estate, and was eventually taken over by a charitable trust. The description makes it sound very like Lundy, but an author's note insists that Combe exists "only in that interesting psychological phenomenon, the imagination of the crime novelist".

A world-famous writer is found hanging from the topmost railings of the lighthouse. His resident editor, his downtrodden daughter and the island's assorted inhabitants form the requisite closed circle of suspects, all shut up together for the duration along with Scotland Yard's Commander Adam Dalgleish and his inspector and sergeant.

This kind of book requires determined suspension of disbelief. A police investigation without teams of technicians seems impossible nowadays, and murder in the middle classes is statistically improbable. Solving a murder among a group of highly educated and articulate people is less likely still, as Dalgleish recognises. "A small group of suspects, if each was intelligent and prudent enough to keep his or her counsel and resist the fateful impulse to volunteer more than was asked, could complicate any investigation and bedevil the prosecution."

Of course Dalgleish always gets his man or woman in the end, and the culprit is usually the least likely person. So far, so traditional. But James uses the familiar framework to build a complicated structure of arguments and insights. She has said that the rigid form of a traditional "golden age" detective novel, like the 14 lines and strict rhyming sequence of a sonnet, sets the writer's imagination free. But she is careful to play fair with the reader, which is a requirement of traditional crime fiction, and, although The Lighthouse is shorter and tauter than some of James's more discursive "middle period" novels, it still goes into meticulous detail about each character's past.

As Dalgleish muses: "A novelist planning to create a fictional Millie for his next novel would know her character better than he knew himself. He would know what she felt and what she thought."

In this novel the fans are already intimately acquainted with the principal players - the detectives themselves. Dalgleish is a published poet as well as a senior policeman: he makes Lord Peter Wimsey-style literary allusions and recognises them too, deducing that a suspect who quotes Bacon has read him. I thought the suspect might just have read the Dorothy L Sayers novel where that particular line was also quoted. Was it a clue? But no; outwitting PD James is more difficult than that.

Whatever Dalgleish looked like when he was a creature only of his author's imagination, Roy Marsden personifies him to any television viewer, played as a man so upright and repressed that it never seems quite credible that his subordinates adore him. This time the enigmatic Dalgleish shows some emotion - the fear of losing his girlfriend, whom he really loves. But he still loves his job more. "It fuelled the poetry. The best of his verse had its roots in the pain, horror and pathetic detritus of the tragic and broken lives, which made up his working life." That bleak realism underlies PD James's work too, enriches it and makes it memorable. Long may she reign.

· Jessica Mann's next crime novel, The Mystery Writer, will be published in 2006 by Allison & Busby